An increasing number of applications, such as digital video satellite broadcast television systems, utilize elliptical antenna reflectors to improve gain and interference rejection in desired directions. This is particularly true for ground-based antenna systems designed to receive from and/or transmit to geo-stationary satellites when other potential interfering satellites are closely spaced, for example on the order of two degrees away. Simply increasing a circular antenna's reception area improves gain and interference rejection in all directions. Increasing the antenna size should also be balanced against cost and aesthetic tradeoffs. Elliptical antenna reflectors strike a better balance between these competing design objectives by increasing the size of the antenna reflector more in the direction in which interference rejection is most critical. The resulting elliptical antennas maintain a relative small reflector size (collection area) while providing improved rejection of unwanted signals in the direction needed. This is typically accomplished by aligning the long axis of the antenna reflector with the geostationary arc. Elliptical reflectors can also be designed to improve the antenna's performance when multiple feeds are used to receive from or transmit to multiple locations (such as multiple satellites).
In general, elliptical antenna feed horns should be used in connection with elliptical reflectors in order to achieve optimum performance. Although elliptical antenna feed horns are somewhat more complex than ordinary circular feeds feed horns, there are a number of established design approaches for elliptical beam feeds. In addition, many applications are now using circular polarity. This is where the challenge arises. It is difficult to achieve good circular polarity cross polarization isolation (also referred to as x-polarization or x-pol isolation) when using an elliptical beam feed with a circular polarity polarizer (also referred to as a CP polarizer) approaches. The problem arises because an elliptical horn (or most any non-axially symmetric horn) introduces a differential phase shift between orthogonal electric fields that are parallel (or near parallel) to either the wide or narrow sides of the horn. The result is that when a circular polarity signal is received by an elliptical horn the asymmetries in the horn introduce a phase differential between the orthogonal fields, changing the circular polarity into elliptical polarity at the output of the horn. Simply attaching a conventional CP polarizer to a feed horn with an elliptical portion results in poor cross-polarization performance due to the differential phase and amplitude characteristics imparted by the elliptical portion of the feed horn.
The following additional background information will facilitate a more detailed discussion of CP polarizers and elliptical antenna feed horns. First it should be appreciated that that circular polarity can be expressed as the vector sum of two orthogonal linear components that are 90 degrees out of phase. For example, the orthogonal linear components can be referred to as +45FV0P (+45 degrees from vertical and 0 degrees phase reference) and −45FV+90P (−45 degrees from vertical and +90 degrees phase). A typical CP polarizer is lined up with the −45LP+90P component and delays that 45FV+90P component by 90 degrees so that it becomes in phase with the +45FV0P component. When this occurs the result is a theoretically lossless conversion of the received power conversion from circular polarity to linear polarity (vertical polarity in this case). This linear polarity can then be easily picked up with simple linear probe, wave-guide slot, etc. If both right hand circular polarity (RHCP) and left hand circular polarity LHCP beams are present, then the CP polarizer produces both vertical and horizontal linear polarity components.
Now consider a theoretically perfect circular polarity beam impinging on an elliptically shaped receiving horn as shown in FIGS. 1A-C. Again, recall that circular polarity can be expressed as the vector sum of two orthogonal linear components that are 90 degrees out of phase. For simplicity in this case, the orthogonal linear components will be taken to be H (horizontal) and V (vertical), where H is aligned with (parallel to) the x-axis, V is aligned with the y-axis, and the z-axis is the signal propagation direction through the horn, as expressed in terms of a conventional Cartesian coordinate system. As the circular polarity beam enters the horn, the elliptical shape of the horn causes the H and V components to travel at different phase velocities through the horn so the H and V components are no longer 90 degrees out of phase when they reach the end of the horn (at the start of the polarizer section). So elliptical polarity now exists at the start of the polarizer section. So a polarizer designed to convert circular polarity to linear polarity will have poor CP cross polarization (cross polarization) performance as shown in FIG. 1D.
As a design compromise, many elliptical reflector systems simply use circular beam feeds with conventional CP polarizers in an attempt to preserve good circular polarity cross polarization isolation. This approach is easy to implement but results in significant compromises (degradations) in efficiency, gain noise temperature, beam width, and side lobe performance of the reflector system, because the circular beam feeds do not properly illuminate the elliptical reflector. This situation is shown in FIG. 2, in which the antenna horn illumination level along the short axis of the reflector is too high resulting in large amounts of wasted spillover energy that degrades gain, efficiency, and noise temperature. In addition, the antenna horn illumination level along the long axis of the reflector is too low resulting in degraded taper efficiency and gain. In addition, this improper illumination makes it very difficult to achieve desired beam width and side lobe performance. That is, the high illumination along the short axis of the antenna degrades (raises) side lobes while the low illumination along the long axis of the antenna degrades (widens) beam widths. In addition, for multi-beam applications where a single reflector is used to receive from multiple beam sources (typically satellites) that are closely spaced, use of a circular feed increases the physical spacing required between the feeds, which limits the closeness of the beams that the antenna can receive.
There has been some work in the area of elliptical beam feed horns that provide circular polarization. U.S. Pat. No. 6,570,542 gives a vague description of an antenna horn that includes a divided elliptical horn section including a phase compensator in the form an “arc structure metal” that spans the entire major axis of the elliptical horn. It is not clear whether or not the “arc structure metal” is used to remove the phase differential introduced by the horn such that a conventional CP polarizer can be attached to it or if the “arc structure metal” is used in conjunction with the horn to achieve the proper phase differentials needed for CP polarizer thereby eliminating the need for a separate CP polarizer. Regardless, this metal structure complicates the manufacturability of the horn making it more difficult to die cast or machine. Also adding the arc through the middle of the horn might require the horn to be wider than desired for many applications.
Accordingly, there is an ongoing need for single and multi-beam elliptical antenna systems that exhibit improved efficiency, gain, interference rejection, gain noise temperature, beam width, side lobe, size and cost and other characteristics.